In the past, there has been proposed an LED lighting device capable of dimming a light emitting diode (LED) light emitting unit (as disclosed in Document 1 (JP 2011-181454 A), for example).
The LED lighting device disclosed in Document 1 includes a constant current output unit and a pulse width modulation (PWM) current output unit. The constant current output unit supplies a first LED current with a constant amplitude to the LED light emitting unit. The PWM current output unit supplies a second LED current with a predetermined duty cycle to the LED light emitting unit. This LED lighting device supplies a current equal to the sum of the first LED current and the second LED current to the LED light emitting unit. When a given dimming level is equal to or higher than a first dimming level, in order to dim the LED light emitting unit, the duty cycle of the second LED current is fixed to a predetermined value, and the amplitude of the first LED current is adjusted according to the given dimming level. When a given dimming level is equal to or lower than a second dimming level lower than the first dimming level, in order to dim the LED light emitting unit, the amplitude of the first LED current is fixed to predetermined amplitude and the duty cycle of the second LED current is adjusted according to the given dimming level.
In the LED lighting device disclosed in Document 1, while the given dimming level is equal to or lower than the second dimming level, the PWM current output unit adjusts the duty cycle of the second LED current according to the given dimming level to thereby dim the LED light emitting unit. Therefore, when the dimming level is changed to decrease luminance, the duty cycle is decreased according to the dimming level, and accordingly, an on-period of the second LED current is shortened. When the on-period of the second LED current is shortened, effects caused by the slew rate will increase, and this may lead to an increase in difficulty of producing a desired LED current, and additionally a decrease in controllability of the dimming control.